A variety of cover assemblies have been developed to prevent particulate matter escaping from open topped truck bodies. Generally, these assemblies comprise a pair of similar arms, pivotally attached to the approximate midpoint of the lower truck body sides. A cover is located between the upper ends of the arms, and means are provided for causing the arms to move pivotally and draw the cover lengthwise across and over the truck body. These cover assemblies are not entirely satisfactory due to the fixed length of the swinging arms once attached to the truck body. Some available assemblies incorporate arms which are axially adjustable to adjust the length of the arms to accomodate different length truck bodies, but do not provide for extension or retraction of the arms during operation of the assembly. U.S. Pat. Nos. 4,050,734 and 4,341,416 disclose such static adjustable arms. The result is a high clearance of the truck body by the cover as the cover is drawn arcuately lengthwise across the truck body. The highest clearance occurs when the swinging arms reach a vertical position. This high clearance is disadvantageous as the cover may be caught by the wind or other objects and be damaged or cause damage. In addition, the assembly may only be used where there is sufficient vertical clearance to permit operation of the similar swinging arms. Another disadvantage is that the assembly may be employed only where attachment of the swinging arms to the approximate midpoint of the side of the truck body is possible.